Epitaxial structure

ABSTRACT

An epitaxial structure includes a substrate, a nucleation layer, a buffer layer, and a nitride layer. The nucleation layer is disposed on the substrate, and the nucleation layer consists of a plurality of regions in a thickness direction, wherein a chemical composition of the region is Al(1−x)InxN, where 0≤x≤1. The buffer layer is disposed on the nucleation layer, and a thickness of the nucleation layer is less than a thickness of the buffer layer. The nitride layer is disposed on the buffer layer, wherein a roughness of a surface of the nucleation layer in contact with the buffer layer is greater than a roughness of a surface of the buffer layer in contact with the nitride layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwan applicationserial no. 107126691, filed on Aug. 1, 2018. The entirety of theabove-mentioned patent application is hereby incorporated by referenceherein and made a part of this specification.

BACKGROUND Technical Field

The disclosure relates to a semiconductor structure, and moreparticularly, to an epitaxial structure.

Description of Related Art

Since the film formed by the epitaxy process has advantages of highpurity, good thickness control, etc., it has been widely applied to theproduction of radio-frequency (RF) components or power components.

In the epitaxial structure adopted in the general RF component, analuminum nitride (AlN) layer as a nucleation layer is formed on theSi-substrate before the epitaxial process is performed. However,spontaneous polarization induced by the nucleation layer material itselfoften occurs at the interface between the nucleation layer and theSi-substrate, or lattice mismatch between the nucleation layer and thesubstrate may induce piezoelectric polarization, which results in thepresence of a parasitic channel. As a result, the RF loss is increased.In addition, the aluminum atoms in the nucleation layer are prone todiffusion to the surface of the Si-substrate, which results in formationof a highly conductive layer, generates leakage current, and affects theRF component characteristics.

SUMMARY

The disclosure provides an epitaxial structure which can solve thediffusion of metal atoms in a nucleation layer to a substrate as in aconventional epitaxial structure, reduce the RF loss, and thus does notaffect the RF component characteristics.

An epitaxial structure of the disclosure includes a substrate, anucleation layer, a buffer layer, and a nitride layer. The nucleationlayer is disposed on the substrate. The nucleation layer consists of aplurality of regions in a thickness direction, and a chemicalcomposition of the regions is Al_((1−x))In_(x)N, where 0≤x≤1. The bufferlayer is disposed on the nucleation layer, and the nitride layer isdisposed on the buffer layer. A thickness of the nucleation layer isless than a thickness of the buffer layer. A roughness of a surface ofthe nucleation layer in contact with the buffer layer is greater than aroughness of a surface of the buffer layer in contact with the nitridelayer.

In an embodiment of the disclosure, a maximum value of the x value inthe plurality of regions is the same, a minimum value of the x value isthe same, and an absolute value of a gradient slope of each of theregions is 0.1%/nm to 50%/nm.

In an embodiment of the disclosure, a maximum value of the x value inthe plurality of regions decreases along the thickness direction, aminimum value of the x value in the plurality of regions is the same, anabsolute value of a gradient slope of each of the regions is 0.1%/nm to50%/nm, and a stepwise slope of the plurality of regions is −0.1%/loopto −50%/loop.

In an embodiment of the disclosure, a maximum value of the x value inthe plurality of regions decreases along the thickness direction, aminimum value of the x value in the plurality of regions is the same,and the x value in the chemical composition of each four of the regionsconsists of four sections of variation, the four sections of variationincluding: a first gradient region gradually changing from a maximumvalue to a minimum value, a second gradient region gradually changingfrom the minimum value to the maximum value, a third gradient regiongradually changing from the maximum value to the minimum value, and afourth gradient region gradually changing from the minimum value to themaximum value. The maximum value of the x value in the first gradientregion, the second gradient region, and the third gradient region is thesame, the maximum value of the x value of the fourth gradient region isa value that decreases from the maximum value of the x value of thefirst gradient region at a stepwise slope of −0.1%/loop to −50%/loop,and an absolute value of a gradient slope of each of the regions is0.1%/nm to 50%/nm.

In an embodiment of the disclosure, the x value in the chemicalcomposition of each of the regions increases or decreases periodicallyat a gradient along the thickness direction.

In an embodiment of the disclosure, the x value in the chemicalcomposition of each four of the regions consists of four sections ofvariation along the thickness direction, the four sections of variationincluding: a first fixed region of a maximum value, a first gradientregion gradually changing from the maximum value to a minimum value, asecond fixed region of the minimum value, and a second gradient regiongradually changing from the minimum value to the maximum value. Anabsolute value of a gradient slope of the first gradient region and thesecond gradient region is 0.1%/nm to 50%/nm.

In an embodiment of the disclosure, the x value in the chemicalcomposition of the plurality of regions decreases stepwise along thethickness direction, the x value in each of the regions does not changealong the thickness direction, and a stepwise slope of the plurality ofregions is −0.1%/loop to −50%/loop.

In an embodiment of the disclosure, the x value in the chemicalcomposition of each of the regions decreases at a stepwise gradientalong the thickness direction.

In an embodiment of the disclosure, a maximum value of the x value inthe plurality of regions decreases along the thickness direction, andthe x value in the chemical composition of each two of the regionsconsists of a fixed region and a gradient region. A gradient slope ofthe gradient region is −0.1%/nm to −50%/nm, and a stepwise slope of thefixed regions in the plurality of regions is −0.1%/loop to −50%/loop.

In an embodiment of the disclosure, the x values in the chemicalcomposition of each two of the regions are respectively two fixed valuesand are stacked along the thickness direction.

In an embodiment of the disclosure, the x value decreases linearly alongthe thickness direction, and a gradient slope of the x value is −0.1%/nmto −50%/nm.

In an embodiment of the disclosure, the x value remains unchanged alongthe thickness direction, and the x value is not smaller than 10%.

In an embodiment of the disclosure, an initial content of the x value ofthe nucleation layer is 10% to 100%, an end content of the x value is 0%to 90%, and an initial content of the (1−x) value is 0% to 90%, and anend content of the (1−x) value is 10% to 100%.

Based on the above, in the epitaxial structure of the disclosure, thenucleation layer includes a plurality of regions, the chemicalcomposition of each of the regions is Al_((1−x))In_(x)N, and the x valuehas different types of variation along the thickness direction.Therefore, it is possible to solve the issue of the presence of theparasitic channel resulting from large spontaneous polarization as wellas piezoelectric polarization generated by lattice mismatch in thenucleation layer and the Si-substrate in the conventional epitaxialstructure. Moreover, it is also possible to solve the issue of reducedinterface resistance caused by the diffusion of atoms of the nucleationlayer to the Si-substrate, which further reduces the RF loss withoutaffecting the RF component characteristics.

To make the aforementioned more comprehensible, several embodimentsaccompanied with drawings are described in detail as follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the disclosure, and are incorporated in and constitutea part of this specification. The drawings illustrate exemplaryembodiments of the disclosure and, together with the description, serveto explain the principles of the disclosure.

FIG. 1 is a schematic cross-sectional view of an epitaxial structureaccording to an embodiment of the disclosure.

FIG. 2 is a schematic view showing a nucleation layer in a fixed contentvariation in an epitaxial structure according to the above embodiment.

FIG. 3 is a schematic view showing a nucleation layer in a gradientcontent variation in an epitaxial structure according to the aboveembodiment.

FIG. 4 is a schematic view showing a nucleation layer in a stepwisecontent variation in an epitaxial structure according to the aboveembodiment.

FIG. 5 is a schematic view showing a nucleation layer in a stepwisegradient content variation in an epitaxial structure according to theabove embodiment.

FIG. 6 is a schematic view showing a nucleation layer in a periodiccontent variation in an epitaxial structure according to the aboveembodiment.

FIG. 7 is a schematic view showing a nucleation layer in a periodicgradient content variation in an epitaxial structure according to theabove embodiment.

FIG. 8 is a schematic view showing a nucleation layer in a fullygradient content variation in an epitaxial structure according to theabove embodiment.

FIG. 9 is a schematic view showing a nucleation layer in another fullygradient content variation in an epitaxial structure according to theabove embodiment.

FIG. 10 is a schematic view showing a nucleation layer in still anotherfully gradient content variation in an epitaxial structure according tothe above embodiment.

DESCRIPTION OF THE EMBODIMENTS

Some embodiments are provided hereinafter and described in detail withreference to the accompanying drawings. However, the embodimentsprovided are not intended to limit the scope of the disclosure.Moreover, the drawings are only descriptive and are not drawn to scale.

To facilitate understanding, the same components will hereinafter bedenoted by the same reference numerals.

FIG. 1 is a schematic cross-sectional view of an epitaxial structureaccording to an embodiment of the disclosure.

Referring to FIG. 1, the epitaxial structure of the present embodimentincludes a substrate 100, a nucleation layer 102, a buffer layer 104, anitride layer 106, and a barrier layer 108. The material of thesubstrate 100 is, for example, Si, Al₂O₃, SiC, GaAs, or other suitablematerial.

The nucleation layer 102 is disposed on the substrate 100. The thicknessof the nucleation layer 102 is less than the thickness of the bufferlayer 104, and the roughness of a surface 102 a of the nucleation layer102 in contact with the buffer layer 104 is greater than the roughnessof a surface 104 a of the buffer layer 104 in contact with the nitridelayer 106. The nucleation layer 102 consists of a plurality of regions110 in the thickness direction. The chemical composition of theplurality of regions 110 is Al_((1−x))In_(x)N, where 0≤x≤1. The x valuerepresents the In (indium) content, and the (1−x) value represents theAl (aluminum) content. In addition, in the description herein, a“region” is defined as a variation in the x value, but the number of theregions does not necessarily represent the number of layers. Due to themanufacturing process, one single layer structure may contain multiplevariations in the x value. Therefore, one single layer may be composedof one single or a plurality of regions 110. Although four regions 110are shown in FIG. 1, the disclosure is not limited thereto. In otherembodiments, the number of the regions 110 is, for example, 2 to 100, orpreferably, 2 to 20. In this range, more desirable interface quality andtwo-dimensional electron gas (2DEG) characteristics can be obtained. Theroughness (rms) of the surface 102 a of the nucleation layer 102 incontact with the buffer layer 104 is generally between 1 nm and 10 nm,or preferably, between 1 nm and 3 nm. In this range, more desirableinterface quality and 2DEG characteristics can be obtained. Thethickness of the nucleation layer 102 is, for example, 1 nm to 500 nm,or preferably, 1 nm to 200 nm. In this range, more desirable interfacequality and 2DEG characteristics can be obtained. In the presentembodiment, by disposing the nucleation layer 102, it is possible toreduce the amount of piezoelectric and spontaneous polarization, improvethe highly conductive layer caused by the parasitic channel, reduce thestress of the epitaxial structure, adjust the warpage of the epitaxialstructure after epitaxial growth, and improve the crack length.

The buffer layer 104 is disposed on the nucleation layer 102. Thematerial of the buffer layer 104 is, for example, aluminum nitride(AlN). The roughness (rms) of the surface 104 a of the buffer layer 104in contact with the nitride layer 106 is, for example, between 0.2 nmand 3 nm and is less than the roughness of the surface 102 a of thenucleation layer 102, which will contribute to subsequent epitaxialgrowth of the nitride layer 106. The thickness of the buffer layer 104is generally greater than 500 nm. In the present embodiment, the bufferlayer 104 is disposed in the epitaxial structure as a stresscompensation adjustment and can adjust the warpage of the epitaxialstructure after epitaxial growth to thereby improve the crack length.The nitride layer 106 is disposed on the buffer layer 104. The materialof the nitride layer 106 is, for example, gallium nitride (GaN),aluminum nitride (AlN), indium nitride (InN), or aluminum gallium indiumnitride (AlGnInN). In addition, in an embodiment, the epitaxialstructure may also be provided with the barrier layer 108 on the nitridelayer 106. The material of the barrier layer 108 is, for example,aluminum nitride (AlN), indium nitride (InN), aluminum gallium indiumnitride (AlGnInN), or aluminum indium nitride (AlInN).

FIG. 2 to FIG. 10 are schematic views showing various content variationsof a nucleation layer in an epitaxial structure of the above embodiment,where “content” refers to the x value.

FIG. 2 is a schematic view showing a nucleation layer in a fixed contentvariation. Referring to FIG. 2, the fixed content variation is definedas a variation in which the x value in the chemical compositionAl_((1−x))In_(x)N of the plurality of regions 110 remains unchangedalong the thickness direction of the nucleation layer, and the x valueis not smaller than 10% (i.e., the indium content ratio is not smallerthan 10%), or preferably not smaller than 60%. In this range, moredesirable interface quality and 2DEG characteristics can be obtained.The thickness of the nucleation layer is, for example, 1 nm to 500 nm,or preferably 1 nm to 200 nm. The surface roughness of the nucleationlayer is, for example, 1 nm to 10 nm, or preferably 1 nm to 3 nm. Theprocess temperature of the nucleation layer is generally 500° C. to 850°C., or preferably 500° C. to 700° C. In this range, more desirableinterface quality and 2DEG characteristics can be obtained.

FIG. 3 is a schematic view showing a nucleation layer in a gradientcontent variation. Referring to FIG. 3, the gradient content variationis defined as a variation in which the x value in the chemicalcomposition Al_((1−x))In_(x)N in the plurality of regions 110 decreaseslinearly along the thickness direction, and the gradient slope is, forexample, −0.1%/nm to −50%/nm, or preferably −0.5%/nm to −10%/nm. In thisrange, more desirable interface quality and 2DEG characteristics can beobtained. Moreover, the initial content of the x value is, for example,10% to 100%, or preferably 50% to 100%. In this range, more desirableinterface quality and 2DEG characteristics can be obtained. The endcontent of the x value is, for example, 0% to 90%, or preferably 0% to50%. In this range, more desirable interface quality and 2DEGcharacteristics can be obtained. The initial content of the (1−x) valueis, for example, 0% to 90%, or preferably 0% to 50%, and the end contentof the (1−x) value is, for example, 10% to 100%, or preferably 50% to100%. The thickness of the nucleation layer is, for example, 1 nm to 500nm, or preferably 1 nm to 200 nm, and the surface roughness of thenucleation layer is, for example, 1 nm to 10 nm, or preferably 1 nm to 3nm.

In another embodiment, the x value has an initial content of 100% and anend content of 0%. The (1−x) value has an initial content of 0% and anend content of 100%.

FIG. 4 is a schematic view showing a nucleation layer in a stepwisecontent variation. Referring to FIG. 4, the stepwise content variationis defined as a variation in which the x value in the chemicalcomposition Al_((1−x))In_(x)N in the plurality of regions 110 decreasesstepwise along the thickness direction, the x value does not changealong the thickness direction in each of the regions 110, and thestepwise slope of the plurality of regions 110 is −0.1%/loop to−50%/loop, or preferably −0.1%/loop to −20%/loop. In this range, moredesirable interface quality and 2DEG characteristics can be obtained.Herein, the term “loop” means that there are two different contents(i.e., high and low contents) that are stacked periodically. Moreover,the initial content of the x value is, for example, 10% to 100%, orpreferably 50% to 100%, and the end content of the x value is, forexample, 0% to 90%, or preferably 0% to 50%. The initial content of the(1−x) value is, for example, 0% to 90%, or preferably 0% to 50%, and theend content of the (1−x) value is, for example, 10% to 100%, orpreferably 50% to 100%. The thickness of the nucleation layer is, forexample, 1 nm to 500 nm, or preferably 1 nm to 50 nm. The number of theregions 110 in the nucleation layer is 2 to 100, or preferably 2 to 20.In this range, more desirable interface quality and 2DEG characteristicscan be obtained. The surface roughness of the nucleation layer is, forexample, 1 nm to 10 nm, or preferably 1 nm to 3 nm.

FIG. 5 is a schematic view showing a nucleation layer in a stepwisegradient content variation. Referring to FIG. 5, the stepwise gradientcontent variation is defined as a variation in which the maximum valueof the x value in the chemical composition Al_((1−x))In_(x)N in theplurality of regions 110 decreases along the thickness direction, andthe x value in the chemical composition of each two regions 110 consistsof a fixed region and a gradient region. In other words, each tworegions 110 are composed of a fixed region having the x value as a fixedvalue and a gradient region having the x value decreasing linearly. InFIG. 5, the x value of the fixed regions decreases stepwise, and thestepwise slope of the fixed regions is, for example, −0.1%/loop to−50%/loop, or preferably −0.1%/loop to −20%/loop. The gradient slope ofthe gradient regions is, for example, −0.1%/nm to −50%/nm, or preferably−0.5%/nm to −10%/nm.

In the stepwise gradient variation, the initial content of the x valueis, for example, 10% to 100%, or preferably 50% to 100%, and the endcontent of the x value is, for example, 0% to 90%, or preferably 0% to50%. The initial content of the (1−x) value is, for example, 0% to 90%,or preferably 0% to 50%, and the end content of the (1−x) value is, forexample, 10% to 100%, or preferably 50% to 100%. The thickness of thenucleation layer is, for example, 1 nm to 500 nm, or preferably 1 nm to50 nm, and the number of the regions 110 of the nucleation layer is, forexample, 2 to 100, or preferably 2 to 20. The surface roughness of thenucleation layer is, for example, 1 nm to 10 nm, or preferably 1 nm to 3nm.

FIG. 6 is a schematic view showing a nucleation layer in a periodiccontent variation. Referring to FIG. 6, the x values in the chemicalcomposition of each two regions 110 are respectively two fixed valuesand are stacked along the thickness direction. The initial content ofthe x value is, for example, 10% to 100%, or preferably 50% to 100%, andthe end content of the x value is, for example, 0% to 90%, or preferably0% to 50%. Herein, the term “initial content” refers to the contact endbetween the nucleation layer and the substrate as the initial position,and “end content” refers to the contact end between the nucleation layerand the buffer layer as the end position. The initial content of the(1−x) value is, for example, 0% to 90%, or preferably 0% to 50%, and theend content of the (1−x) value is, for example, 10% to 100%, orpreferably 50% to 100%. The thickness of the nucleation layer is, forexample, 1 nm to 500 nm, or preferably 1 nm to 50 nm, and the number ofthe regions 110 of the nucleation layer is, for example, 2 to 100, orpreferably 2 to 20. The surface roughness of the nucleation layer is,for example, 1 nm to 10 nm, or preferably 1 nm to 3 nm.

FIG. 7 is a schematic view showing a nucleation layer in a periodicgradient content variation. Referring to FIG. 7, the periodic gradientcontent variation is defined as a variation in which the x valueincreases or decreases periodically and gradually along the thicknessdirection. For example, the x value in the chemical compositionAl_((1−x))In_(x)N of each four regions 110 in the nucleation layerconsists of four sections of variation along the thickness direction,and the four sections of variation include: a first fixed region havinga fixed value which is the maximum value, a first gradient regiongradually changing from the maximum value to the maximum value, a secondfixed region having another fixed value which is the minimum value, anda second gradient region gradually changing from the minimum value tothe maximum value. The absolute value of the gradient slope of the firstgradient region and the second gradient region is, for example, 0.1%/nmto 50%/nm, or preferably 0.5%/nm to 10%/nm.

In the periodic gradient variation, the initial content of the x valueis, for example, 10% to 100%, or preferably 50% to 100%, and the endcontent of the x value is, for example, 0% to 90%, or preferably 0% to50%. The initial content of the (1−x) value is, for example, 0% to 90%,or preferably 0% to 50%, and the end content of the (1−x) value is, forexample, 10% to 100%, or preferably 50% to 100%. The thickness of thenucleation layer is, for example, 1 nm to 500 nm, or preferably 1 nm to50 nm, and the number of the regions 110 of the nucleation layer is, forexample, 4 to 100, or preferably 4 to 20. The surface roughness of thenucleation layer is, for example, 1 nm to 10 nm, or preferably 1 nm to 3nm.

FIG. 8, FIG. 9, and FIG. 10 are schematic views showing a nucleationlayer in three fully gradient content variations.

The fully gradient content variation is defined as a variation in whichthe x value in the chemical composition of each of the regions 110increases or decreases fully gradually along the thickness direction.

Referring to FIG. 8, the maximum value of the x value in the regions 110is the same, and the minimum value of the x value is the same. Moreover,the absolute value of the gradient slope of each of the regions 110 is,for example, 0.1%/nm to 50%/nm, or preferably 0.5%/nm to 10%/nm. Theinitial content of the x value is, for example, 10% to 100%, orpreferably 50% to 100%, and the end content of the x value is, forexample, 0% to 90%, or preferably 0% to 50%. The initial content of the(1−x) value is, for example, 0% to 90%, or preferably 0% to 50%, and theend content of the (1−x) value is, for example, 10% to 100%, orpreferably 50% to 100%. The thickness of the nucleation layer is, forexample, 1 nm to 500 nm, or preferably 1 nm to 50 nm. The number of theregions 110 of the nucleation layer is, for example, 2 to 100, orpreferably 2 to 20. The surface roughness of the nucleation layer is,for example, 1 nm to 10 nm, or preferably 1 nm to 3 nm.

In FIG. 9, the maximum value of the x value in the plurality of regions110 decreases along the thickness direction, and the minimum value ofthe x value is the same. The absolute value of the gradient slope ofeach of the regions 110 is, for example, 0.1%/nm to 50%/nm, orpreferably 0.5%/nm to 10%/nm, and the absolute value of the stepwiseslope of the regions 110 is, for example, 0.1%/loop to 50%/loop, orpreferably 0.5%/loop to 10%/loop. The initial content of the x value is,for example, 10% to 100%, or preferably 50% to 100%, and the end contentof the x value is, for example, 0% to 90%, or preferably 0% to 50%. Theinitial content of the (1−x) value is, for example, 0% to 90%, orpreferably 0% to 50%, and the end content of the (1−x) value is, forexample, 10% to 100%, or preferably 50% to 100%. The thickness of thenucleation layer is, for example, 1 nm to 500 nm, or preferably 1 nm to50 nm, and the number of the regions 110 of the nucleation layer is, forexample, 2 to 100, or preferably 2 to 20. The surface roughness of thenucleation layer is, for example, 1 nm to 10 nm, or preferably 1 nm to 3nm.

In FIG. 10, the maximum value of the x value in a plurality of regions110 a to 110 d decreases along the thickness direction, the minimumvalue of the x value is the same, and the x value in the chemicalcomposition of the four regions 110 a to 110 d consists of four sectionsof variation. The four sections of variation include: the first gradientregion 110 a gradually changing from the maximum value to the minimumvalue, the second gradient region 110 b gradually changing from theminimum value to the maximum value, the third gradient region 110 cgradually changing from the maximum value to the minimum value, and thefourth gradient region 110 d gradually changing from the minimum valueto the maximum value. The maximum value of the x value is the same inthe first gradient region 110 a, the second gradient region 110 b, andthe third gradient region 110 c. The maximum value of the x value in thefourth gradient region 110 d is a value that decreases from the maximumvalue of the x value of the first gradient region 110 a (the secondgradient region 110 b and the third gradient region 110 c) at a stepwiseslope of −0.1%/loop to −50%/loop, and the stepwise slope is preferably−0.5%/loop to −10%/loop. The absolute value of the gradient slope ofeach of the regions 110 a to 110 d is, for example, 0.1%/nm to 50%/nm,or preferably 0.5%/nm to 10%/nm.

In FIG. 10, the initial content of the x value is, for example, 10% to100%, or preferably 50% to 100%, and the end content of the x value is,for example, 0% to 90%, or preferably 0% to 50%. The initial content ofthe (1−x) value is, for example, 0% to 90%, or preferably 0% to 50%, andthe end content of the (1−x) value is, for example, 10% to 100%, orpreferably 50% to 100%. The thickness of the nucleation layer is, forexample, 1 nm to 500 nm, or preferably 1 nm to 50 nm. The number of theregions 110 a to 110 d of the nucleation layer is, for example, 4 to100, or preferably 4 to 20. In this range, more desirable interfacequality and 2DEG characteristics can be obtained. The surface roughnessof the nucleation layer is, for example, 1 nm to 10 nm, or preferably 1nm to 3 nm.

From the perspective of solving the RF loss, the embodiments of FIG. 8to FIG. 10 are preferred. The reason is that since the interface of thenucleation layer and the buffer layer is a continuous variation, thedefect density of the interface can be reduced, and the quality of theepitaxial material and the interface smoothness can be improved. Inaddition, since the interface is a continuous variation, the interfacestress generated is small, and the amount of polarization is relativelylow, which can effectively improve the highly conductive layer caused bythe parasitic channel and thereby reduce the RF loss.

In summary of the above, according to the epitaxial structure of thedisclosure, the issue of the conventional nucleation layer (AlN) can beimproved through the different content (x value) variations in thenucleation layer of Al_((1−x))In_(x)N. Table 1 below shows the expectedRF characteristics of the conventional nucleation layer and thenucleation layer of the disclosure applied to the epitaxial structure.

TABLE 1 Conventional Nucleation layer nucleation of the disclosure layerAlN Al_((1−x))In_(x)N Lattice matching Inferior Superior Spontaneouspolarization High Low Charge induced by spontaneous Much Littlepolarization Piezoelectric polarization High Low Charge induced bypiezoelectric Much Little polarization Total amount of polarization HighLow Al diffusion coefficient in High Low Si-substrate

According to Table 1, since the nucleation layer of the disclosure hasbetter lattice matching with the Si-substrate, lower spontaneouspolarization, and lower piezoelectric polarization, and can reduce thediffusion of aluminum in the Si-substrate, it is possible to reduce theRF loss and ensure the RF characteristics.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the disclosed embodimentswithout departing from the scope or spirit of the disclosure. In view ofthe foregoing, it is intended that the disclosure covers modificationsand variations provided that they fall within the scope of the followingclaims and their equivalents.

What is claimed is:
 1. An epitaxial structure comprising: a substrate; anucleation layer disposed on the substrate and being in direct contactwith the substrate, wherein the nucleation layer consists of a pluralityof regions in a thickness direction, and a chemical composition of theplurality of regions is Al_((1−x))In_(x)N, where 0≤x≤1, wherein amaximum value of the x value in the plurality of regions decreases alongthe thickness direction, a minimum value of the x value in the pluralityof regions is the same, an absolute value of a gradient slope of each ofthe regions is 0.1%/nm to 50%/nm, and a stepwise slope of the pluralityof regions is −0.1%/loop to −50%/loop; a buffer layer disposed on thenucleation layer, wherein a thickness of the nucleation layer is lessthan a thickness of the buffer layer; and a nitride layer disposed onthe buffer layer, wherein a roughness of a surface of the nucleationlayer in contact with the buffer layer is greater than a roughness of asurface of the buffer layer in contact with the nitride layer.
 2. Theepitaxial structure according to claim 1, wherein an initial content ofthe x value of the nucleation layer is 10% to 100%, an end content ofthe x value is 0% to 90%, and an initial content of the (1−x) value is0% to 90%, and an end content of the (1−x) value is 10% to 100%, whereinthe initial content of the x value is located on a bottom portion of thenucleation layer close to the substrate, and the end content of the xvalue is located on a top portion of the nucleation layer close to thebuffer layer.
 3. An epitaxial structure comprising: a substrate; anucleation layer disposed on the substrate and being in direct contactwith the substrate, wherein the nucleation layer consists of a pluralityof regions in a thickness direction, and a chemical composition of theplurality of regions is Al_((1−x))In_(x)N, where 0≤x≤1, wherein amaximum value of the x value in the plurality of regions decreases alongthe thickness direction, a minimum value of the x value in the pluralityof regions is the same, and the x value in the chemical composition ofeach four of the regions consists of four sections of variation, thefour sections of variation comprising: a first gradient region graduallychanging from a maximum value to a minimum value, a second gradientregion gradually changing from the minimum value to the maximum value, athird gradient region gradually changing from the maximum value to theminimum value, and a fourth gradient region gradually changing from theminimum value to the maximum value, wherein the maximum value of the xvalue in the first gradient region, the second gradient region, and thethird gradient region is the same, the maximum value of the x value ofthe fourth gradient region is a value that decreases from the maximumvalue of the x value of the first gradient region at a stepwise slope of−0.1%/loop to −50%/loop, and an absolute value of a gradient slope ofeach of the regions is 0.1%/nm to 50%/nm; a buffer layer disposed on thenucleation layer, wherein a thickness of the nucleation layer is lessthan a thickness of the buffer layer; and a nitride layer disposed onthe buffer layer, wherein a roughness of a surface of the nucleationlayer in contact with the buffer layer is greater than a roughness of asurface of the buffer layer in contact with the nitride layer.
 4. Theepitaxial structure according to claim 3, wherein an initial content ofthe x value of the nucleation layer is 10% to 100%, an end content ofthe x value is 0% to 90%, and an initial content of the (1−x) value is0% to 90%, and an end content of the (1−x) value is 10% to 100%, whereinthe initial content of the x value is located on a bottom portion of thenucleation layer close to the substrate, and the end content of the xvalue is located on a top portion of the nucleation layer close to thebuffer layer.